The present invention relates to a method for fabricating a semiconductor of group III-V nitrides constituting a semiconductor laser device of which application to the optical information processing field and the like is expected, a method for fabricating a semiconductor substrate, and semiconductor light emitting devices fabricated using such methods.
Group III-V nitride semiconductors using nitrogen (N) as a group V element have received attention as promising materials for short-wavelength light emitting devices because they have a comparatively large band gap. Among others, gallium nitride (GaN) based compound semiconductors (AlxGayInzN (0 less than xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, x+y+z=1)) have been studied vigorously, and blue light emitting diode (LED) devices and green LED devices made of GaN-based semiconductors have already been commercialized.
To increase the storage capacity of optical disc devices, semiconductor laser devices having an oscillating wavelength in the 400 nm band are eagerly demanded. In this relation, semiconductor laser devices made of GaN-based compound semiconductors have received attention, and are now about to reach a level of commercialization.
A GaN-based semiconductor laser device has a device structure generally formed by growing crystals on a substrate made of sapphire (Al2O3 single crystal), silicon carbide (SiC), or the like by metal-organic vapor phase epitaxy (MOVPE).
A conventional GaN-based semiconductor laser device will be described with reference to FIG. 8. FIG. 8 shows a cross-sectional construction of a conventional GaN-based semiconductor laser device capable of providing laser oscillation.
As shown in FIG. 8, on a substrate 101 made of sapphire, formed sequentially by crystal growth are a low-temperature growth buffer layer 102, a strain suppression layer 103 made of n-type Al0.05Ga0.95N, an n-type cladding layer 104 made of n-type Al0.07Ga0.93N, an n-type optical guide layer 105 made of n-type GaN, a multiple quantum well (MQW) active layer 106 made of GaInN, a block layer 107 made of p-type AlGaN, a ptype optical guide layer 108 made of p-type GaN, a P-type cladding layer 109 made of p-type Al0.07Ga0.93N, and a p-type contact layer 110 made of p-type GaN.
A feature of the above conventional semiconductor laser device is the strain suppression layer 103 formed on the low-temperature growth buffer layer 102. The strain suppression layer 103 is made of Al0.05Ga0.95N. The mole fraction of Al of this composition, 0.05, is determined to be a value close to the Al mole fraction of the n-type cladding layer 104 made of Al0.07Ga0.93N, of which the lattice constant is smallest among the plurality of semiconductor layers constituting the laser structure. The strain suppression layer 103 having this composition serves to reduce strain as the underlying layer of the n-type cladding layer 104. Thus, with the existence of the strain suppression layer 103, it is possible to reduce occurrence of cracking in the cladding layer 104 or warping of the substrate 101 that may be caused by crystal strain during the formation of the laser structure.
The n-type and p-type cladding layers 104 and 109 have a thickness of about 0.5 xcexcm, the largest among the layers of the laser structure, and also have the largest Al mole fraction among the layers because they must secure a large band gap and a small refractive index. Therefore, cracking generally tends to occur in the cladding layers.
To overcome the above problem, in the conventional semiconductor laser device, the Al mole fraction of the strain suppression layer 103 is simply determined so that the lattice constant of the strain suppression layer 103 is a value somewhere between the lattice constant of the substrate 101 made of sapphire and that of the cladding layers 104 and 109 made of AlGaN.
Another crystal growth method is also reported where a substrate made of gallium nitride formed by hydride vapor phase epitaxy (H-VPE) or the like is used as the substrate 101 in place of the sapphire substrate.
However, in the above conventional semiconductor growth method, the lattice constants of the strain suppression layer 103 and the cladding layers 104 and 109, which are both made of AlGaN, are not determined by strict designing, but determined by simply setting the Al mole fraction of the strain suppression layer 103 at a value close to that of the cladding layers 104 and 109. Therefore, when the temperature is lowered to room temperature after the crystal growth, the strain suppression layer 103 undergoes strain due to a difference in thermal expansion coefficient between the substrate 101 and the strain suppression layer 103 and therefore changes in lattice constant. As a result, the lattice constant of the strain suppression layer 103 differs from that of the cladding layers 104 and 109, and this causes occurrence of cracking or warping.
In the case of the substrate 101 made of gallium nitride, also, in which the lattice constant is decisively different between the cladding layers and the substrate, cracking or warping occurs in the cladding layers.
An object of the present invention is to ensure that no cracking or the like occurs particularly in a semiconductor layer having a small lattice constant among a plurality of layered semiconductor layers made of group III-V nitrides.
To attain the above object, in a structure using a substrate made of a material different from a group III-V nitride, the lattice constant of a semiconductor layer having a comparatively small lattice constant among a plurality of semiconductor layers grown on the substrate, that is, a semiconductor layer containing aluminum, is made to substantially match with the lattice constant of a strain suppression layer at room temperature after thermal shrinkage or thermal expansion.
In a structure using a substrate made of a group III-V nitride, the lattice constant of a semiconductor layer having a comparatively small lattice constant among a plurality of semiconductor layers grown on the substrate, that is, a semiconductor layer containing aluminum, is made to substantially match with the lattice constant of the substrate.
The first method for fabricating a semiconductor of the present invention includes the steps of: (1) growing a first semiconductor layer made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61) on a substrate at a temperature higher than room temperature; and (2) growing a second semiconductor layer made of AluGavInwN (0 less than uxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61, u+v+w=1) over the first semiconductor layer, wherein in the step (1), the mole fraction x of Al of the first semiconductor layer is set so that the lattice constant of the first semiconductor layer at room temperature substantially matches with the lattice constant of the second semiconductor layer in the bulk state after thermal shrinkage or thermal expansion.
According to the first method for fabricating a semiconductor of the present invention, cracking and the like will not occur in the aluminum-containing second semiconductor layer having a comparatively small lattice constant even when the temperature is lowered to room temperature after the growth of the second semiconductor layer.
Preferably, the first method for fabricating a semiconductor further includes the step of growing a third semiconductor layer having an Al mole fraction smaller than the second semiconductor layer between the first semiconductor layer and the second semiconductor layer or over the second semiconductor layer. With this construction, the third semiconductor layer can function as an active layer including a quantum well layer. Thus, the second semiconductor layer having an Al mole fraction larger than the third semiconductor layer can function as a cladding layer.
In the first method for fabricating a semiconductor, the substrate preferably is composed of sapphire, silicon carbide, or silicon. This ensures growth of a semiconductor of a group III-V nitride.
The second method for fabricating a semiconductor of the present invention includes the step of growing a semiconductor layer made of AlxGavInwN (0 less than uxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61, u+v+w=1) over a semiconductor substrate made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61), wherein the lattice constant of the semiconductor substrate is made to substantially match with the lattice constant of the semiconductor layer in the bulk state.
According to the second method for fabricating a semiconductor of the present invention, occurrence of cracking and the like in the semiconductor layer is prevented.
The third method for fabricating a semiconductor of the present invention includes the step of growing a semiconductor layer made of AluGavInwN (0 less than uxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61, u+v+w=1) over a semiconductor substrate made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61).
In the third method for fabricating a semiconductor, the semiconductor substrate preferably contains indium.
The method for fabricating a semiconductor substrate of the present invention includes the step of forming a semiconductor substrate from AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61) to be used as a substrate over which a semiconductor layer made of AluGavInwN (0 less than uxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61, u+v+w=1) is grown, wherein the mole fraction x of Al of the semiconductor substrate is set so that the lattice constant of the semiconductor substrate substantially matches with the lattice constant of the semiconductor layer in the bulk state.
According to the method for fabricating a semiconductor substrate of the present invention, cracking and the like are prevented from occurring in a semiconductor layer grown on the semiconductor substrate.
The first semiconductor light emitting device of the present invention includes: a first semiconductor layer made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61) formed on a substrate; and a second semiconductor layer made of AluGavInwN (0 less than uxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61, u+v+w=1) formed over the first semiconductor layer, wherein the lattice constant of the first semiconductor layer at room temperature substantially matches with the lattice constant of the second semiconductor layer in the bulk state after thermal shrinkage or thermal expansion.
According to the first semiconductor light emitting device of the present invention, cracking and the like are prevented from occurring in the second semiconductor layer that contains aluminum and thus has a comparatively small lattice constant.
Preferably, the first semiconductor light emitting device further includes an active layer having an Al mole fraction smaller than the second semiconductor layer between the first semiconductor layer and the second semiconductor layer or over the second semiconductor layer, wherein the second semiconductor layer is a cladding layer.
In the first semiconductor light emitting device, the substrate preferably is composed of sapphire, silicon carbide, or silicon.
The second semiconductor light emitting device of the present invention includes: a semiconductor substrate made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61); and a semiconductor layer made of AluGavInwN (0 less than uxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61, u+v+w=1) formed over the semiconductor substrate, wherein the lattice constant of the semiconductor substrate substantially matches with the lattice constant of the semiconductor layer in the bulk state.
According to the second semiconductor light emitting device of the present invention, occurrence of cracking and the like in the semiconductor layer is prevented.
The third semiconductor light emitting device of the present invention includes: a semiconductor substrate made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61); and a semiconductor layer made of AluGavInwN (0 less than uxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61, u+v+w=1) formed over the semiconductor substrate.
In the third semiconductor light emitting device, the semiconductor substrate preferably contains indium.